Aluminum is light in weight and easy to form as compared to steels and has been favorably used in the field of containers. Specifically, aluminum has been widely used for producing seamless cans through severe workings of forming such as drawing and ironing.
In the seamless aluminum cans mentioned above, in general, chromium type surface treating films are formed by conversion treatment on the inner and outer surfaces of the aluminum sheet that forms the cans in order to improve corrosion resistance of aluminum. Further, such seamless aluminum cans are formed through severe working of forming. Therefore, polyester resin coating or films are, usually, formed on the inner and outer surfaces of the cans via the surface-treating films.
However, to meet strong demands concerning environmental sanitation in recent years, study has been conducted extensively in connection with non-chromium inorganic surface-treating films for use as films for conversion-treating the surfaces of aluminum. For instance, a patent document 1 is proposing a resin-coated aluminum alloy can lid having surface-treating films formed on the surfaces of the aluminum by the conversion treatment by using a zirconium compound.
However, the non-chromium inorganic surface-treating film proposed above does not fully adhere to the resin coating that is formed thereon, and leaves room for improvement if it is to be applied to the seamless cans that are formed through severer working of forming than the working for forming the can lids. That is, in the resin-coated aluminum seamless cans having a resin coating formed on the non-chromium inorganic surface-treating film, adhesion is not sufficient between the resin coating and the surface-treating film. Therefore, fine scratches are formed on the outer surface during the working for forming the cans or during the working (such as necking or double-seam working) after the cans have been formed. As a result, due to the subsequent thermal hysteresis such as pasteurizing treatment (sterilization by low-temperature heating) after the content has been filled or retort sterilization, the resin coating peels (scar-caused delamination) being triggered by fine scratches and renders the appearance to become defective.
A patent document 2 is a patent application filed by the present applicant and proposes an art of forming an organic/inorganic surface-treating film containing a zirconium compound, a phosphorus compound or an organic compound, as means for solving the problem of when the non-chromium inorganic surface-treating film is applied to the resin-coated aluminum seamless cans. The above surface-treating film adheres to the resin coating to a high degree and, therefore, is capable of effectively preventing the resin coating from peeling despite the outer surface gets scratched during the working for forming the seamless cans or through the working that follows the working of forming.
According to the patent document 2, however, the organic/inorganic composite surface-treating film is formed by using, as a liquid for treating the film, an aqueous solution in which are dissolved or dispersed a zirconium compound as well as an organic compound, requiring a cumbersome operation for controlling the concentration of the treating liquid. Therefore, it is not easy to form the film, an increased cost is required for the production, and further improvements are desired.
Further, if the resin coating is formed on the metal material by extrusion lamination, a problem arouses in that neck-in (a phenomenon in which the film-like resin that is melt-extruded becomes narrower than the width of the die outlet) easily occurs. The neck-in causes irregularity in the thickness of the resin coating on the inner and outer surfaces and, therefore, causes dispersion in the properties such as closely adhering property. This becomes conspicuous particularly when a lowly crystalline polyester is used as the resin.